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By Michael King and David Herring
Reprinted here with permission of the Encyclopedia of Atmospheric Sciences, Edited by Jim Holton, John Pyle, and Judy Curry; published by Academic Press 2002.
The atmosphere changes chemically and physically on widely varying time scalesranging from minutes to decadesand is therefore a challenge to measure precisely over the entire globe. But with the National Aeronautics and Space Administrations (NASA) 1960 launch of the Television Infrared Observation Satellite (TIROS), Earth scientists began a new mission to observe large-scale weather patterns from space. In the late 1970s, their mission expanded to include global-scale measurements that would help them understand the causes and effects of longer-term climate change. NASA and its affiliated agencies and research institutions collaborated to develop a series of research satellites that have enabled scientists to test new remote sensing technologies that have advanced scientific understanding of both chemical and physical changes in the atmosphere. (Remote sensing involves the use of devices other than our eyes to observe or measure things from a distance without disturbing the intervening medium.) The goal is to examine our world comprehensively to determine what dynamics drive our planets climate system and how climate change affects, and is affected by, our environment.
Depending upon their measurement objectives, research satellites primarily fly in one of two orbits: (1) a near-polar, sun-synchronous orbit to allow their senors to observe the entire globe at the same solar time each day, or (2) a mid-inclination, precessing orbit to focus their sensors on the equator and lower latitudes where the observations are made at different times of day to better sample time-varying phenomena such as clouds. Some polar-orbiting satellite sensors can observe any given place on the globe as often as every day, thus collecting data with high temporal (time) resolution. Other satellite sensors view any given place as infrequently as once every 16 days, thus having relatively low temporal resolution for a satellite sensor, but still far surpassing our ability to make these same measurements with surface-based or aircraft sensors. Satellite sensors with high spatial resolution (15 meters per pixel) can discern objects in the atmosphere or on the surface as small as say a football field or farmland, thus providing high spatial resolution. Other satellite sensors that are designed to measure continental and global-scale dynamics typically have only moderate (500 meters per pixel) to low (20 kilometers per pixel) spatial resolution. Satellite sensors carry specially designed detectors that are particularly sensitive to certain wavelengths of the electromagnetic spectrum, called spectral bands. The more precisely a remote sensor can measure narrow bands of radiant energy, and the greater the number of these discreet bands it can measure, the higher is its spectral resolution. The atmosphere interacts with solar radiation much like a venetian blindselectively absorbing and reflecting certain wavelengths of solar energy while allowing others to pass through. Engineers design satellite remote sensors to be particularly sensitive to those wavelengths that can be reflected or emitted back up through the atmosphere to space, thus enabling them to make their measurements.
Earth-orbiting remote sensors provide the best means of collecting the data scientists need because they can measure things on scales of time and space that otherwise would not be possible. Moreover, satellite sensors not only observe wavelengths of visible light, they also precisely measure wavelengths of radiant energy that our eyes cannot see, such as microwaves, ultraviolet rays, or infrared light. If scientists know how certain objects (like cirrus clouds or windblown dust) typically absorb, reflect, and emit particular wavelengths of radiant energy, then by using satellite sensors to precisely measure those specific bands of the electromagnetic spectrum, scientists can learn a lot about the Earths atmosphere and surface. Remote sensors allow us to observe and quantify key climate and environmental vital signs such as temperature, ozone concentrations, carbon monoxide and other pollutants, water vapor and other greenhouse gases, cloud types and total cloud cover, aerosol types and concentrations, radiant energy fluxes, and many more.